


Alone with a Heart

by MarshmarrowSans



Series: Tumblr Requests [5]
Category: Undertale (Video Game)
Genre: (Or as we call them soul cycles...), Accidental Voyeurism, Demisexual Sans, Ecto-Penis (Undertale), F/M, Female Masturbation, Hints of Sub Sans bc I can't keep my hands off that good shit, Male Masturbation, Masturbation, Monster Heat, Mutual Masturbation, Pining, Reader has a vagina, Sensitive bones, Sub Sans, THERE'S A LOT OF MASTURBATION OK, Very Awkward Pining, Virgin sans, awkward pining
Language: English
Status: Completed
Published: 2017-11-18
Updated: 2018-04-27
Packaged: 2019-02-03 18:29:10
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 5,050
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/12753777
Author URL: https://archiveofourown.org/users/MarshmarrowSans/pseuds/MarshmarrowSans
Summary: I received a request to do a little something based on this imagine I did awhile ago:Shortie (MT Sans)- He has no idea what’s happening the first time his soul cycle rolls around, especially because it’s probably, embarrassingly, before he even asks you out.  Why the fuck does he feel like running laps around the property?  Why the fuck can’t he stop thinking about hugging you and nuzzling into you and the way you smell and how soft and warm you are?  He’s so high-strung and pissed off.- He can hardly look you in the eyes the next morning because he spent all night jerking off while thinking about you.





	1. His Heart

**Author's Note:**

> https://www.youtube.com/watch?v=oCyVGhUHoOs ;)

Shortie felt... off all day. He was used to always feeling sluggish, always dragging himself from task to task. But today he rolled out of bed fresh as a daisy. It was kind of nice, at first. It got him through his work with a speed and efficiency unbefitting of his usual demeanor. 

That wasn't all. He felt... affectionate. Weirdly affectionate. He stared at you longingly from across the table at every meal you spent with the crew, which probably looked funny, because he was also stuffing his face. But he couldn't keep his eyes or his mind off you, and god knew if only you'd let him, he wouldn't be able to keep his hands off you, either. He just wanted to hug you and never let go. Was that weird? Very. Especially since he'd spent the last few months being bitter towards you and treating you like dirt for being a human. But damn, if it didn't make him happy to think about.

But by the end of the day, almost paradoxically, he felt both energized and irritated. Almost like he was overstimulated by his own nerves, frustrated, pent up, and needing to do... something. He didn't know what. No amount of angry pacing made it go away. He felt like his magic was over-exerting itself without reason. He wasn't entirely wrong.

He'd heard vaguely of soul cycles before, but considering he didn't believe in soulmates, he didn't believe in them, either. Because he'd heard it said that when a soul recognized its mate in another person, it would periodically flood itself with magic and begin a natural process to, ahem, urge its owner to procreate with their soulmate, every month or so until fertilization occurred. He'd been pretty sure that was just a myth that arose out of somebody making fun of human hormonal cycles, and he forgot about it. But if only he remembered it, he would be certain of its existence now, because his soul felt like it was on fire, and if this wasn't his magic overflowing at the thought of you, he didn't know what was.

It was like puberty missed him on the first swing and then hit him, all at once, twice as hard with the backhand. He'd never really had urges like this, and just figured he never would. Asexuality was A-okay, as he always said. Now he was feeling like he ought to recategorize himself somewhere in the gray-asexual spectrum. But that was a conundrum for another time. He was alone in his room, alone with his thoughts of you and a soul that was begging him, begging his entire body to indulge feelings for you he never thought he'd have for anyone, and the last thing he wanted to do was try and untangle his own complicated feelings. All he could think to do right now was act on them.

He paced until he felt like he was sweltering in his suit and vest. And then, somewhat angrily, somewhat desperately, somewhat passionately, he took them off. He could feel his magic concentrating in two regions of his body: his chest, he expected, but... between his legs? That was just lewd. And yet what usually would make him feel disgusted just sent a rush of adrenaline and magic through him. He felt almost euphoric, and he knew just what would tip him into being completely so.

His hands fumbled clumsily at his belt as he opened the floodgates, so to speak, and let his mind fill with thoughts of you. He could hardly focus on a single thing; it was everything that he secretly admired about you, all at once. Your eyes. Your lips. Your smile. The way your hair shined in the sunlight, the way you seemed to glow in the moonlight. The way you tried and tried to be his friend and never gave up, no matter how much shit he gave you for it. The way you believed in him like nobody else ever could.

He wondered what you would think of him if you could see him right now, clambering into his bed completely naked, his whole body demanding you this instant, even when he knew that he couldn't have you. In his right mind, he would've figured you would think he was pathetic. Even impassioned as he was, he still knew for a fact that he was pretty pathetic right now. But your kindness and your endless patience with him was fresh in his mind right now, amplified, even, and all he could imagine was that you would understand. You would forgive him, and... you would want to help him.

He flipped over on his stomach and stifled a soft moan in his pillow. He wasn't even touching himself, not with his hands, but he was so hot and bothered that his crotch and tailbone rubbing against the sheets were enough to sufficiently stimulate him. The reality hit him that he was about to masturbate to thoughts of his first crush. Well, this was an awkward moment almost 30 years in the making. No sense in stopping now, especially since he knew he wouldn't get a wink of sleep if he didn't get some relief, first.

Luckily, he had two pillows, and so he kept one where it was to support his head, and he pinned the other one under his hips. That was much better than just the bedsheets. Slowly, experimentally, he dragged his hips forward. The pillow was so soft, and it yielded to his movement, just like he imagined your body would.

It kept going back to that. You, you, you. He could hardly separate the concept of feeling good and the thought of you in his head right now. You were like a drug, more addictive than his favorite cigars. He rocked back, then forward again. How could he possibly feel both worse and better when he did that? That awful, wonderful tension between his legs only intensified, but it felt like it was going somewhere.

Slowly, with his warm face nestled into the pillow that he wasn't currently humping, he built up a steady tempo. It was easier that way, with regular, unfocused movements, to think about you rather than what he was doing. He thought about how comforting you were. He thought about how wonderful it would be to be wrapped in your arms right now. He always resisted your hugs in the past, but he would give anything for you to give him one right now, for you to pull him close to you, take care of his needy soul and body and tell him that you would make everything alright--

His first orgasm hit him, so sudden and so intense and so confusing that he yelped more than he moaned, though the ecstasy settled in after a few seconds of shaky, uncoordinated thrusting. He felt like his legs would give out under him. He hadn't been expecting it (or himself) to come so fast, but he wasn't about to resist it. He rode it out as long as it would go, his moans turning to soft, relieved laughter and his hips teetering back to that slow, experimental pace he'd started out with. The thought of your hands on his hips, guiding him through it, followed all the way through, from that first, intense wave to the final remnants of the last. Oh, fuck, yes. Was he supposed to stop there? He wasn't planning on stopping there.

He didn't know why he was breathing so hard. He felt completely energized. He flipped over on to his back and held the pillow between his legs, on top of him. That position was more befitting of you, now that he thought about it. In this world, it was plow or be plowed, and honestly, you out-plowed him. And that didn't hurt him one bit. If he could just have you now, riding his hips with his hands on yours... he couldn't imagine anything better.

This time, he thought about the few times you laid your hands on him. They were tiny, insignificant moments to you, he was sure. But they were the world to him. He knew you got the wrong idea when he tensed up from your touches. It was true, from most people, he hated physical contact. But he flinched from you because he was shocked that he loved it so much. That time you hugged him because he made you laugh so hard, that time you sarcastically patted his cheek after making a particularly scathing comment at him, that time he came home, miserable and dust-covered, and you just quietly rubbed between his shoulder blades until he started to feel better... He hadn't been ready to admit it at the time, and still wasn't ready to admit it out loud, but here, to himself, he could. He loved it when you touched him. He fucking craved it. He'd never craved it as much as he did right now.

He wanted you on top of him. He wanted you to touch him on his aching soul and on his aching crotch. He wanted you to ride him while your fingers traced delicately through his ribs, wanted you to touch him wherever your curiosity guided you. God knew you'd find some awesome way to make him squeal. You were smart like that. You always seemed to know what you were doing, even when he knew you didn't. You just... caught on to things. And he wanted you to catch on to him.

He could feel himself rapidly getting pent up again, and that only urged him further. He went from rocking his hips up into the pillow to bouncing them, forgetting all about his caution not to make the bed creak too much. He was too absorbed in his fantasy to care. He wouldn't let anything so petty get between him and you. It was an absolutely new sensation, and he found himself digging his fingers into the soft material and wishing it was your ass. It was that last, uncharacteristically lewd thought that sent him into a fit of ecstasy all over again.

Much like he'd stopped giving a damn about making the bed creak, he stopped giving a damn about being too vocal, too. He let out a deep groan and hissed a series of repetitions of your name under his breath in time with his thrusts. He was a little more steady this time, having been taken a little less off-guard than the first time, now that he knew how his own body proceeded through orgasm. Still, it was a fairly new feeling, and after a few moments of strength, he broke down again into sloppy movements and desperate gasps for air.

His mind or his body wanted more. His body or his mind was begging him, in all his exhaustion, to stop for the night and rest. He didn't know which was which anymore, but he listened to the former demand.

His burning soul kept him awake for another hour, floundering desperately around with his pillow and his hands in search of climax after climax, until finally his exhaustion won out and he fell asleep, on his back, with his legs spread and your name still lingering on his tongue.

* * *

The morning after was what could only be described as torture. You were there for breakfast, as you often were, and not only did he feel like your eyes were burning into the back of his neck whenever he wasn't looking. He couldn't bring himself to look you in the eyes, either.

He was disgusted with himself, for a variety of reasons, but even more than the most obvious one, it was because, after spending all night dreaming about all the things that made you so amazing, he knew that now he was basically giving you the cold shoulder. He hoped that you weren't hurt by it. You weren't sensitive, but he knew that he hurt your feelings sometimes, and he felt like such an asshole for it. His... activities, were still too fresh in his mind for him to convincingly act like nothing had happened.

You were alone in the room together. Everybody else had filtered out, while you ate slowly and he just quietly poked at his meal and barely ate. He felt your eyes on him again. He was certain of it. He flushed nervously.

"... what?" It came out sounding so irritated. It wasn't out of character for him, but he still felt bad for it. Especially when you recoiled from him a little bit. Ugh, why did he always have to go and screw things up?

"Look... I don't know how to say this without being awkward, so, I'm just gonna bite the proverbial bullet and say it. I... heard you. Last night."

He felt like his soul just got crushed to pieces in his chest. Oh, no. Oh, shit. His eyes went completely dark, and his face, completely blue.

"... i... don't know what to say," he mumbled. He wanted to cover his non-existent ears and teleport away. He did not want to hear what you had to say next.

"Yeah. I heard you crying. I mean... sobbing, it sounded like. I wanted to make sure you were okay." You paused. He was completely silent while his mind processed that. "... You don't have to talk about it if you don't want to. I just wanted you to know that, if you're ever sad enough that you feel like you have to lock yourself in your room and cry, you can talk to me. Okay?" You smiled, in that dazzling way of yours that drove him wild, and gave him a firm couple of pats on the shoulder. He was still frozen there, hardly able to believe the close call. You didn't know. You didn't know. Oh thank god, you didn't know. Also, you were incredibly sweet and kind to him, as usual, but for once, that was besides the point.

"yeah? yeah. oh man. i was crying... so hard last night." Shortie chuckled nervously, stabbing his fork through a copious amount of steak and eggs. "i, uh, guess i was just thinking about the... the deep stuff."

You raised an eyebrow at him. "The deep stuff...?"

"yeah. like you said, i don't have to talk about it, right? so, uh. i don't really want to talk about it. right now. maybe later."

Much later. Much, much later. If you were ever crazy enough, and if he was ever lucky enough, for you to fall in love with him.


	2. Her Heart

**Summary for the Chapter:**

> I was going to leave this story off at one chapter, buuut...
> 
> Anonymous said to marshmarrowsans:  
> Hey km the anon who asked for a followup to that shortie fic, I was wondering if maybe the crush could be pleasuring themselves with a blue dildo? Just to make all the more awkward when shortie finds them
> 
> This sounded hot as hell, so now it's Reader's turn!

Soul cycles were a monthly thing, and considering his muted endogenous sex drive, Shortie was getting used to it being the only time that he felt such an intense physical urge to be with you.

 

This wasn’t, of course, to say that those periodic, mating behavior-related fluctuations in his magic were the only thing driving his attraction to you.  Nothing could be further from the truth.  He knew it for a fact that it was his pre-existing feelings for you that facilitated that attraction—if he didn’t already have strong, strong feelings for you, then your soul wouldn’t have a strong enough grip on his to launch his magic into a horny frenzy every month, now would it?

 

It was easier to deal with once he figured out the periodicity of it.  He marked it on his calendar, and became acutely aware of when his appetite and energy levels began to spike as a prelude to the main event.  He learned to be more careful, more private, more quiet as to what he was doing on those particular nights alone in his room.  It was embarrassing, but it was normal for a monster involved in a soul bond, and it was manageable.

 

That is, it was manageable until you completely unintentionally knocked him off-kilter again, as you had a tendency to do.

 

To be fair, it was probably more his fault for trying to enter your room without knocking than it was your fault for failing to lock your door.

 

He’d taken in a breath to announce his presence, had to be milliseconds away from doing so, when he cared to look over at where he at first only processed vaguely that you were lying down on your bed.  He had to hit the emergency brakes on his voice, because his visual processing caught up to the situation and he soon realized that you were… indecent.  
  
_Very_ indecent.  
  
Namely, you were naked from the waist down, legs spread and bent at the knee, situated at just the right angle so that he could see the area between your thighs in all its alien beauty.  It really did look like something straight out of a sci-fi story—he was so flustered by what he saw at first and such a certified nerd that _that_ was the first thing to sprung to mind.  If he could be honest, he never considered that humans might have significant patches of hair anywhere but their head.  From there, he could see your exposed folds, spread open for a brief moment when your fingers formed a V shape, then obscured again when you plunged both fingers into yourself.  You were very wet, and it coated your fingers with a noticeable sheen.

 

Impulsively, he almost teleported away, which probably would have been the safest best to avoid detection completely, but instead, he took a swift step to the side to hide himself behind the doorframe and take a moment to collect himself.  Two sides of him were in conflict right now.  First, there was the side of him he was most familiar with.  The one that…  _should_ have been unaffected by this.  It was still fresh in his mind that he’d never had much of an interest in sex in the grand scheme of things.  He knew that if you were anybody else, or that even if you were yourself at an earlier point in time, he would’ve announced his presence like it was no big deal, stood in the doorway with his arms crossed and a huge grin on his face while you blushed and scrambled for your clothes and yelled at him to get the fuck out.  And, of course, he would have left you to get your bearings and never brought it up again, save a few raunchy jokes and jabs at your bedtime habits.  
  
But then, that was when he would’ve been able to brush this sort of thing off.  It would be hard to act so non-chalant about something that affected him so greatly, as it did now.  That was where the other part of him came in—the part of him he’d really only seemed to find within himself when his soul cycle started.  The part of him that craved the very thing he once mocked as something dirty and weird and unnecessary.  The part of him that made him spend all night, once a month, indulging in his fantasies about you and touching himself until he was too exhausted to carry on.  The part of him that was curious, almost scientifically so, about you and your body and what you liked.  The part of him that was desperate to know how correct his imagination was.  Were you as dominant as you came off?  How did you like to be touched?  What felt good for you?  How beautiful would you look when you came?

 

Stay or leave.  That was what it came down to.  Stay or leave…

 

He stood with his back to the wall and stared up at the ceiling, letting out a tense sigh.  Ah, shit.  What the hell was he thinking?  Of course he should leave.  What was the alternative?  Stand there and watch you like a creeper?  You weren’t some fucking zoo animal to be observed.  You weren’t his fucking lab rat for figuring out these feelings that only seemed to finally hit him far, far past pubescence, when they’d hit most others.  You were…    
  
You were the love of his life.  
  
He couldn’t do this.  
  
Or so he thought, until he heard you moan, soft as a summer breeze, something that sounded an awful lot like--  
  
“Sans…  fuck, you little cocktease.  Give me what I want already.”  
  
At first, he only heard his name, without anything tacked on at the end.  He was ready to give himself up and try to brush this off as some kind of elaborate joke, and not him lingering at your door trying to figure out what the hell to do with his thirsty self.  But then you repeated it, and you said so much more, and he realized something that made it quite impossible for him to just leave like he’d been planning to do.  
  
You weren’t acknowledging his presence.  You were fantasizing about him.

 

That was a concept that confounded him and made his soul start to throb.  He’d never thought of himself as attractive, in terms of either his personality or his physical appearance, and he’d never minded all that much.  He never minded being ugly, until he had someone he wanted to impress.  The best he was ever expecting was for you to tolerate his low place on the 1-10 scale physically if he cleaned up his act personality-wise, because there was no changing his short stature or the thick build of his body.  


But stars, the way you sighed his name.  The way you demanded from him, in whatever fantasy was going on in your head, that he give you what you want.  You made it sound like he was the only one in the world who could do that for you.  
  
He thought he couldn’t stay?  Fuck that.  Now he couldn’t leave.

 

Besides, from the sound of things, you wanted him there.

 

Finally, he gathered the courage to glance into your room again.  
  
You were very much distracted at the moment.  You’d diverted your attention to something on one of the shelves at your bedside.  He could see, when you pulled it up on to the bed, that it was a small lockbox.  Damn.  He’d seen it there before, briefly figured it had some cash or a gun in it, and didn’t give it any more thought.  Now it turned out it was your little toybox.  Good information to know.

 

He was aching in anticipation, waiting to see what you would use to get yourself off.  He was hoping, since you were doing this solo, it would be the best indication of what felt good for you.  He _had_ to know.  Did you like being penetrated, or did you prefer playing with the more external parts of your anatomy?  Would you use something that vibrated?  Something long or short, thick or thin, curved or straight, smooth or ridged?  
  
Imagine his surprise and delight when, from the box, you produced a dildo that was, most notably, a striking blue color.  It wasn’t a perfect resemblance, of course.  The toy was of a sky blue hue, whereas his was more sapphire.  It was of fairly average length, probably around 5 inches, and average girth, whereas his was shorter and thicker.  But he wasn’t expecting a perfect resemblance, especially since you’d never seen his ecto-cock before.  All that mattered was the thought—and it was a very, very naughty thought of yours.

 

Watching you push the blue length into yourself with such impatience harkened back memories of his own fantasies of you.  He could, quite literally, only imagine how amazing that would feel on his end.  Look at him, jealous over a fuckin’ dildo.  That, right there, was what he wanted, what he craved, more than anything those nights he spent alone.  He felt incredible just from what he could do with his hands and his pillows, but if he had you?  You would be warm, and wet, and tight, and responsive.  You would respond to him, in some ways his mind probably couldn’t even predict.  He could touch you and feel the shape of your body that, up until now, he could only watch longingly through your clothes and ponder about.

 

It was agonizing, in the best way possible, watching the blue appendage sink into you.  You pushed it in until you seemed to hit a certain spot that made you suck in air through your teeth.  He thought you might have hurt yourself, but you hit that spot again, whatever it was, and your back arched in pleasure.  You loved that.  What was it you were doing quite right to make yourself react like that?  He did his best to glimpse how far in you’d inserted it into yourself, and at what angle.  It was a steeper one than he’d expected, and not as far in as he would’ve thought.  If that were actually his cock, he could probably reach that spot no problem, if he buried himself all the way to the bones of his hips in you…

 

Damn.  It wasn’t his time of the month.  Not even close.  But his whole body felt white-hot, his magic was so incredibly excited, it’d take no effort at all to just…

 

He hid himself behind the wall again, back pressed to it and the image of you fresh in his mind, and slipped his hand between his femurs.  In fact, up until this point, it had been taking effort _not_ to form his ecto-cock.  It was a way of concentrating that overflowing magic into something manageable, something he could handle manually.  It was a small relief of tension in itself, forming the appendage between this legs, and he sighed in both relief and anticipation while his hand slid over his bulge.  He had to remind himself to be quiet this time.  Extremely quiet.  He could let out a moan or whine or two when he was in his room, but now he was right outside yours, with the door cracked open.  Masturbating silently wasn’t his forte, but damn it, he was taking the risk.

 

Damn.  How did you turn him into this without even trying?  You weren’t putting on an act, you weren’t doing shit with his pleasure in mind, you were just making yourself feel good, and that was all it took.  That was all it took to make him stand by your door, listening desperately for another sigh or squeal in that sweet voice of yours.  All he could imagine was that blue cock—his or just the imitation of it, it hardly even fucking mattered to him anymore right now—plunging in and out of you, becoming coated in your arousal.  All he could imagine was how good that would feel, to be the one doing that to you, and that he could be, oh, stars, that he _could_ be.  You _wanted_ him to be.  His hand slipped under the waistband of his pants and his boxers to grip his aching length and begin to stroke it.  Not in that lazy way he usually started out with, either—he skipped straight to the action.  Straight to trying to get himself over the edge.

 

He couldn’t have enjoyed this at all if he didn’t know he was on your mind.  He would’ve felt too jealous, unreasonably so, considering he’d hardly treated you amicably in all your time together, and there was certainly no official romance, let alone exclusivity, between you two.  Nevertheless, it made his magic burn in an unpleasant way to think about you thinking about anybody else this way.  Besides that, he would’ve felt like he was intruding on something that wasn’t his.  But this?  This was his.  And that made him yours.  God, did that ever make him yours, even if he couldn’t quite work up the courage to tell you that.  His knees felt weak at the thought of doing absolutely anything to please you, of earning your approval.

 

He heard those beautiful little noises coming from you every once in awhile and could only imagine himself earning them from you.  He wanted to join in with you so badly, like a harmony of the enamored and desperate.

 

Despite you getting quite the head start compared to him, he still found himself reaching his climax almost immediately after yours.  It was as incredible, mind-blowing, earth-shattering as ever, but many times more strange and confusing for him.  How on earth could something so simple and so downright dirty make him FEEL like this?  He didn’t know, but he felt it nonetheless, in the rush and pounding of his magic and the impressive spurts of cum that soaked his hand and the crotch of his pants.  It felt so sinfully good, all other thoughts were temporarily erased from his mind.  He wanted to sob out your name and the fact that he loved you so much, for making him feel this way and for everything else, but he was confined to just clawing the fabric of his shorts and holding his breath until the post-orgasmic sense of relief washed over him.

 

… And the shame hit him pretty quickly after that.  He was angry at himself, for being such a pervert, yes, but even more for that, for not being able to fully commit to it.  Damn it, Shortie!  Be a perverted asshole or be a gentleman, you can’t live with a foot in each world!  He hated that he’d played the part of the voyeur, and he hated that he was left regretting it afterwards.  He would’ve been better off having no regrets about it, or not doing it at all.

 

He’d had this silly idea in his mind, much earlier, that the moment you were finished with yourself, he was going to knock at your door, make you jump a foot in the air, and smugly announce that dinner was ready, which was what he’d come to tell you in the first place.  Now he was in a much more shameful condition than you, and was in no place to do such a thing.  He was absolutely filthy, both in body and soul.  For once in his life, he was of the strong opinion that he needed to take a shower.  
  
…  Maybe he’d treat himself just one more time while he was in there.

**Author's Note:**

> Hit us up at marshmarrowsans.tumblr.com! We love hearing from y'all! Requests, imagines, asks for the mods, anything at all! <3


End file.
